1. Field of the Invention
The present invention is related to a method for manufacturing an organic electroluminescence (EL) device. In particular, the present invention is related to a method for manufacturing an organic EL device by a relief printing method.
2. Description of the Related Art
In an organic EL device, a light-emitting layer comprising an organic light-emitting material is formed between a pair of opposing electrodes, and light emission is induced by passing an electric current to the light-emitting layer. In order to emit light with good efficiency, the thickness of the light-emitting layer plays an important role and the layer has to be formed as a thin film with a thickness of about 100 nm. Further, it has to be patterned with a high degree of accuracy to produce a display.
A low molecular material and a high molecular material can be used as the organic light emitting material for forming the light emitting layer. The low molecular material is subjected to resistive heating vapor deposition and the like to form a thin film, and at the same time the patterning is performed by using a microscopically patterned mask. However, this method has a problem in that patterning accuracy is reduced with an increase in the size of a substrate.
Therefore, the high molecular material has recently been used as the organic light emitting material, and a method of forming a thin film by wet coating with a coating liquid obtained by dispersing or dissolving the organic light emitting material into a solvent has been attempted. As the wet coating method for the thin film formation, spin coating, bar coating, projection coating, dip coating, and the like are known. However, the wet coating methods have difficulty in realizing the high definition patterning and color coding with R, G, and B, and it is considered that the thin film is most effectively formed by a printing process that is capable of achieving excellent color coding and patterning.
Further, among various printing processes, the gravure printing and like methods in which a hard plate such as a metal printing plate is used are not suitable for the organic EL device and a display that have a glass substrate. The offset printing using an elastic rubber blanket and a relief process using an elastic rubber plate or resin plate are appropriate for the organic EL device and a display. As attempts on the printing processes, a method employing the offset printing (see Patent Publication 1) and a method employing the relief process (see Patent Publication 2) and so on have been proposed.
Meanwhile, the high molecular organic light emitting material has a low solubility to water and alcohol-based solvents, and it is necessary to use an organic solvent for obtaining a coating liquid (hereinafter referred to as ink) from the high molecular organic light emitting material. An aromatic system organic solvent is suitably used as the organic solvent. Therefore, the ink made from the organic light emitting material (hereinafter referred to as organic EL ink) is an ink of organic solvent.
However, the rubber blanket used in the offset printing is subject to swelling and deformation due to the organic solvent such as toluene or xylene. Though various rubbers such as an olefin-based rubber and a silicone-based rubber can be used for the blanket, the rubbers do not have resistance to the toluene solvent, the xylene solvent, and the like and are subject to swelling and deformation. Therefore, the rubbers are inadequate for printing the organic EL ink.
Also, a flexographic printing process using a plate made from a rubber and a resin relief printing process using a resin plate without a rubber component are included in the relief printing process using an elastic relief plate, and a process using a water-developable resin relief plate, the plate having a water-solubility resin as a main component, is highly resistant to the organic solvents and can be used for the organic EL ink printing.
Due to the above reasons, the relief process using the water-developable photosensitive resin relief plate is the most suitable method of printing the organic EL ink made from the aromatic solvent such as toluene and xylene on the hard substrate such as the glass substrate.
An inking roller or an anilox roll is used for a general resin relief printing method using a resin relief printing plate. In an inking roller method, an ink having a relatively high viscosity is kneaded by a plurality of inking rollers. Ink transfer is repeated several times. Finally, an amount of an ink supplied to a relief printing plate becomes uniform. An anilox roll method is as follows: fine cells are formed on a surface of a roll; a liquid ink is supplied to the cell; an excess ink is removed by a doctor; thereby, a constant amount of an ink is supplied to a printing plate.
On the other hand, as mentioned above, the solubility of an organic light emitting material in an organic solvent is high. Especially, the solubility of an organic light emitting material in an aromatic system organic solvent is high. Therefore, an organic light emitting material is generally used as an aromatic system organic solvent ink. However, among aromatic system organic solvents, toluene or xylene has an aromatic ring with a relatively simple additional group. Therefore, as for toluene or xylene, the solubility of an organic light emitting material is high. Therefore, the concentration of solid content in an ink tends to be easily set. Separating or gelling of an organic light emitting material does not tend to occur. Further, among aromatic system organic solvents, the vapor pressure of toluene or xylene is high. That is, toluene or xylene is a solvent which easily dries. Therefore, a drying temperature can be set relatively low and because the amount of solvent remaining in a light emitting layer is small, toluene or xylele is preferred for an ink.
However, in the case where the vapor pressure is high (highly-volatile), an ink is easily dried during printing. In a relief printing, ink transfer occurs twice. That is, an ink is transferred from an anilox roll or an ink roll to a printing plate, and an ink is transferred form a printing plate to a substrate. In the case of a relief printing, if an ink is easily dried, an ink is easily dried during transfer of an ink. Therefore, a printing failure may occur. Therefore, an idea is proposed in which a printing failure due to drying is prevented by using a solvent having a low vapor pressure (patent document 3).
An ink in which a polymer organic light emitting material is dissolved in a solvent is used for an organic EL ink which is used for forming an organic light emitting layer by a relief printing method. However, if the solubility of an organic light emitting material is low, separating or gelling of a solid component occurs. Then, as mentioned above, if an aromatic system organic solvent is used as a solvent, the solubility of an organic light emitting material becomes high. Thereby, an ink is obtained in which separating or gelling of a solid component does not easily occur. In this case, as for toluene or xylene having a relatively high vapor pressure among aromatic system organic solvents, especially the solubility of an organic light emitting material is high. The solid content can be widely adjusted. In addition, since the vapor pressure is high, a drying temperature can be low while the amount of remaining solvent is small. Therefore, toluene or xylene is preferred. However, as also mentioned above, because an ink including toluene or xylene as a solvent dries quickly, ink drying easily occurs during transfer of the ink, causing a printing failure.
Then, the objective of the present invention is to provide a method for manufacturing an organic EL device. In the method, a light emitting layer can be preferably formed by a relief printing method. In the method, a printing method is used in which a printing failure due to drying of an ink does not occur even if an aromatic system organic solvent such as toluene or xylene having a vapor pressure over 500 Pa at a general room temperature, in which a solubility of an organic light emitting material is high, is used.
In order to achieve the objective, the inventors found that the following method is effective. In the method, a printing failure due to drying of an ink does not occur even if an aromatic system organic solvent such as toluene or xylene having a vapor pressure over 500 Pa at a usual room temperature (20-25 degrees Celsius), in which solubility of a polymer organic EL material is high, is used.    [Patent document 1] JP-A-2001-93668    [Patent document 2] JP-A-2001-155858    [Patent document 3] JP-A-2001-155861